Until now, increasing of the capacity of various optical discs has been realized in such a way that the size of light-condensing spot is reduced on a focal plane, by downsizing information pits formed on the disc tracks, as well as by adopting, for recording and playing back use, a laser beam with a shorter wavelength and an objective lens with a larger numerical aperture.
For example, in a CD (Compact Disc) system, the disc substrate serving as a light transmitting layer (a transparent cover layer and a spacer layer provided on an information recording layer, which are also called as a transparent substrate) is approximately 1.2 mm in thickness, the laser beam wavelength is approximately 780 nm, the numerical aperture (NA) of the objective lens is 0.45, and the recording capacity of the CD is 650 MB. Meanwhile, in a DVD (digital versatile disc), the disc substrate serving as its light transmitting layer is approximately 0.6 mm in thickness, the laser beam wavelength is approximately 650 nm, the NA is 0.6, and the recording capacity of the disc is 4.7 GB. In DVD, two substrates whose thickness is, for example, 0.6 mm are bond together to be used as a 1.2 mm-thickness disc.
In a higher-density BD (Blu-ray Disc), a large memory capacity over 23 GB has been realized using an optical disc with a thinner light transmitting layer of 0.1 mm, by adopting a laser beam wavelength of approximately 405 nm and a NA of 0.85.
In addition to the discs described above, in an HDDVD (high definition digital versatile disc), a large memory capacity over 18 GB has been realized by using an optical disc substrate which serves as light transmitting layer and whose thickness is 0.6 mm as thick as the DVD, and by adopting a laser beam wavelength of approximately 405 nm and a NA of 0.65. Moreover, optical disc technologies are expected to further increase the density higher than those described above.
One of the technologies for increasing data density on optical discs is a super-resolution technology by which recording marks or information pits are formed on an optical disc in a size smaller than the diffraction limit and by which data is played back from the marks or the pits formed on the optical disc.
Generally, in a playback method in which a light beam with a wavelength λ is used and focused with a numerical aperture NA to produce a light spot, it is impossible to read data when a mark pitch (or a pitch for information pit trains) is less than or equal to λ/(2×NA); therefore, the pitch is referred to as the diffraction limit. Assuming that a recording mark portion (an information pit portion) has the same length as a spacer portion within one pitch, the diffraction limit of the recording mark length (or the information pit length) is given as λ/(4×NA).
A super-resolution technology applied for recording and playback includes, for example, a technique in which nonlinear absorbing material that changes the refractive index or transmittance according to the light intensity is used for the optical disc medium to record therein marks or information pits smaller than the diffraction limit using light with locally intensified distribution; and a technique in which metal plasmon effect or other light enhancement effect is additionally given to produce much highly intensified light in order to record marks or information pits smaller than the diffraction limit (for example, techniques described in Patent Document 1, Patent Document 2, Patent Document 3, Non-Patent Document 1, and Non-Patent Document 2).
An optical disc medium structure has been devised as another conventional technique (for example, Non-Patent Document 4) to further increase density in directions orthogonal (hereinafter, referred to as “radial direction”) to track extending directions: information pits smaller than the diffraction limit are arranged as a group track, and diffracted light due to a radially oriented structure with periodically spaced group tracks of the small pits and approximately flat portions therebetween is detected in radial direction as a push-pull signal; thereby, controlling a light-condensing spot, using tracking error signals with respect to the group tracks obtained from the push-pull signal, to accurately track each of pit trains arranged within an interval of the diffraction limit and play back data of the desired small pits.
More specifically, a fundamental structure of the super-resolution optical disc comprises a disc substrate that is provided with small pits smaller than the diffraction limit and on which formed is a film of light-absorbing material causing super-resolution phenomena, such as Silver-Indium-Antimony-Tellurium (AgInSbTe). On the disc, three information-pit trains configured only with the small pits, form a group track, which is regarded as a track having a width broader than the diffraction limit. It has been reported (for example, in Non-Patent Document 4) that a tracking error signal can be obtained by detecting, as a push-pull signal, diffracted light due to the structure described above, and that in order to scan the three pit trains in the group track with a light-condensing spot, the light-condensing spot is moved onto a desired pit train by adding an electrical offset signal to the tracking error signal.
The conventional technique described above can record 1.5 times more densely than that without group tracks; however, in an optical disc medium including pit trains with their width narrower than the diffraction limit, it has been difficult to detect a tracking error signal and take tracking-servo control while further radially increasing its recording density on the optical disc medium.
Patent Document 1: Japanese Patent Laid-Open No. 2004-87073 (page 9, FIG. 1)
Patent Document 2: Japanese Patent Laid-Open No. 2004-235259 (pages 4-5, FIG. 1)
Patent Document 3: Japanese Patent Laid-Open No. 2005-339795 (page 12, FIG. 2)
Non-Patent Document 1: J.J.A.P. Vol. 43, No. 7A, 2004, pp. 4212-4215, “Observation of Eye Pattern on Super-Resolution Near-Field Structure Disk with write-Strategy Technique”
Non-Patent Document 2: J.J.A.P. Vol. 39, Part 1, No. 2B, 2000, pp. 980-981, “A Near-Field Recording and Readout Technology Using a Metallic Probe in an Optical Disk”
Non-Patent Document 3: J.J.A.P. Vol. 45, No. 2B, 2006, pp. 1379-1382, “High-Speed Fabrication of Super-Resolution Near-Field Structure Read-Only Memory Master Disc using PtOx Thermal Decomposition Lithography”
Non-Patent Document 4: ODS Proceedings, WB4, pp. 203-205, “Super-RENS ROM Disc with Narrow Track Pitch”